Central collisions of 16 0 nuclei with the 107 Ag and 80 Br nuclei in nuclear emulsion at 14.6, 60, and 200 GeV/nucleon are compared with proton-emulsion data at equivalent energies. The multiplicities of produced charged secondaries are consistent with the predictions of superposition models. At 200 GeV/nucleon the central particle pseudorapidity density is 58 ± 2 for those events with multiplicities exceeding 200 particles.PACS numbers: 25.70.NpThe recent surge of interest in relativistic nucleusnucleus {A-A) collisions has been driven by the possibility of observing a new state of matter, 1 a quark-gluon plasma. The critical energy density for quark-gluon plasma formation, e^2 GeV/fm 3 , may be reached in central A-A collisions. In contrast, the conservative view is that A-A interactions can be explained as the superposition of many nucleon-nucleon (N~N) interactions in which a nucleus can be approximated as a cluster of free nucleons. 2,3 This view has evolved from the study of p~A collisions, for which superposition models have been shown to provide an adequate explanation of the experimental data. 4 The acceleration of beams of heavy ions up to 200 GeV/nucleon allows this question to be addressed experimentally. Here we present results on particle production in central collisions of 16 0 with Ag/Br nuclei, O-Ag/Br interactions, compare them to central collisions of protons with Ag/Br and to inclusive proton-emulsion (pemul) interactions at similar energies, 5 and examine the consequences of interpreting these data in terms of models that describe A-A collisions as the superposition of TV-TV collisions.Stacks of BR-2 emulsion pellicles with dimensions 5x10 cm 2 x600 fim were exposed horizontally at the Brookhaven National Laboratory to the 14.6-GeV/ nucleon 16 0 beam and at CERN to 60-and 200-GeV/nucleon 16 0 beams. The emulsions were developed and TABLE I. Central O-Ag/Br interactions with TV/ , > 15 and TVF'-O. Energy (GeV/nucleon) 14.6 60 200 Number of events, TV ev 75 123 120 n s D(n s )then scanned with optical microscopes. An along-thetrack scan located 1855 inelastic events, giving an interaction mean free path of 12.0 ±0.3 cm, which corresponds to a total inelastic cross section of 1052 ± 26 mb. This is in good agreement with the calculated value of 12.2 cm (cr = 1040 mb), 6 indicating the high efficiency of the scanning. Only events accompanied by a high excitation of the target nucleus were analyzed. An excited target nucleus evaporates low-energy fragments which produce heavily ionizing tracks (TV;,) that are easily distinguished from the n s relativistic secondaries, for which /< 1.4/ m i n , corresponding to pion energies above 70 MeV and proton energies above 400 MeV. To provide a sample of small impact parameter, or "central," interactions occurring with the Ag or Br nuclei in emulsion, events with TV/ , > 15 and noZ>2 fragments from the incident nucleus (TVp-0) were selected. These central collisions represent (17 ±2)% of the total inelastic cross section and (31 ± 3)% of the interactions wit...
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